1. Field of the Invention
This invention relates generally to analog signal processing, and, more particularly, to improving conversion of voltage signals into current signals.
2. Description of the Related Art
In communications systems, particularly telephony such as a Plain Old Telephone System (POTS), it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire, bi-directional communication channel. A line card generally connects the subscriber station to the central switching office. The functions of the line card include supplying talk battery, performing wake-up sequences of circuits to allow communications to take place, and the like. There are a plurality of signals that are sent and received in a voltage-signal format. Many times, these voltage signals are converted to current signals before being processed.
POTS was designed primarily for voice communication, and thus provides an inadequate data transmission rate for many modem applications. To meet the demand for high-speed communication, designers have sought innovative and cost-effective solutions that would take advantage of the existing network infrastructure. Several technological solutions proposed in the telecommunications industry use the existing network of telephone wires. A promising one of these technologies is the xDSL technology.
xDSL is making the existing network of telephone lines more robust and versatile. Once considered virtually unusable for broadband communications, an ordinary twisted pair equipped with DSL interfaces can transmit video, television, and very high-speed data. The fact that more than six hundred million telephone lines exist around the world is a compelling reason for these lines to be used as the primary transmission conduits for at least several more decades. Because DSL utilizes telephone wiring already installed in virtually every home and business in the world, it has been embraced by many as one of the more promising and viable options.
There are now at least three popular versions of DSL technology, namely Asymmetrical Digital Subscriber Line (ADSL), Very High-Speed Digital Subscriber Line (VDSL), and Symmetric Digital Subscriber Line (SDSL). Although each technology is generally directed at different types of users, they all share certain characteristics. For example, all four DSL systems utilize the existing, ubiquitous telephone wiring infrastructure, deliver greater bandwidth, and operate by employing special digital signal processing. Because the aforementioned technologies are well known in the art, they will not be described in detail herein.
DSL and POTS technologies can co-exist in one line (e.g., also referred to as a xe2x80x9csubscriber linexe2x80x9d). Traditional analog voice band interfaces use the same frequency band, 0-4 Kilohertz (KHz), as telephone service, thereby preventing concurrent voice and data use. A DSL interface, on the other hand, operates at frequencies above the voice channels, from 25 KHz to 1.1 Megahertz (MHz). Thus, a single DSL line is capable of offering simultaneous channels for voice and data. It should be noted that the standards for certain derivatives of ADSL are still in definition as of this writing, and therefore are subject to change.
DSL systems use digital signal processing (DSP) to increase throughput and signal quality through common copper telephone wire. It provides a downstream data transfer rate from the DSL Point-of-Presence (POP) to the subscriber location at speeds of up to 1.5 mega-bits per second (MBPS). The transfer rate of 1.5 MBPS, for instance, is fifty times faster than a conventional 28.8 kilobits per second (KBPS).
DSL systems generally employ a signal detection system that monitors the telephone line for communication requests. More specifically, the line card in the central office polls the telephone line to detect any communication requests from a DSL data transceiver, such as a DSL modem, located at a subscriber station. There are multiple types of signals that are received and transmitted over multiple signal paths during telecommunication operation. Many times it is advantageous to transmit signals in a voltage format, such as to reduce transmission power consumption. However, it is often desirable to convert the voltage signal to current signals before processing the signals. One advantage of processing current signals instead of voltage signals is reduced distortion levels and reduced voltage swings.
Turning now to FIG. 1, one example of a prior art circuit for converting a voltage signal into a current signal, is illustrated. The circuit illustrated in FIG. 1 is a differential circuit, which converts a voltage signal V1 into a current signal, Iout. The voltage at node 10 is equal to the input voltage V1. The voltage at node 11, V2, is a voltage level that differs from the input voltage by xcex94V, where xcex94V is:
xcex94V=V1xe2x88x92V2.
Ideally, the input voltage V1 is converted to the output current Iout, proportional to V1, as defined by the equation:
xe2x80x83Iout=V1/R.
However, due to non-linearites that are typically present in devices such as transistors Q1 and Q2, Iout generally does not equal to V1 divided by R. The current flow through Q1 (through a line 7) is approximately equal to
[I/2]+xcex94I.
The current flow through Q2 (through a line 9) is approximately equal to
[I/2]xe2x88x92xcex94I.
xcex94I ideally can be defined by the equation
xcex94I=xcex94V/[2*R].
However, the transistors Q1 and Q2, which in one embodiment are bi-polar junctions transistors (BJT), contain an inherent resistance at the emitter 17, called the emitter resistance (ReQ1, ReQ2). Therefore, in reality, the current differential, xcex94I, is equal to
xcex94V/[2*(R+ReQ1)].
Due to the non-linear effects of the transistors Q1, Q2 shown in FIG. 1, to receive acceptable signal integrity when converting a voltage signal to a current signal, the current Iout has to be much greater than the voltage differential xcex94V divided by 2*R
(i.e., Iout greater than  greater than xcex94V/[2*R]).
If small-level signals are used, the non-linear effects of the transistors, Q1, Q2 (i.e. effects such as the emitter resistance causing variations in the voltage), can cause significant noise-levels in the converted signal. Furthermore, it is often desirable to process small-level signals in communication circuits.
When large-level voltage signals are converted to large-level current signals, the non-linear effects of the transistor are reduced. However, using large-level signals consumes more power in electronic circuitry. Furthermore, large level signals that are sent through amplification circuits tend to cause a larger amplification of noise levels, resulting in large overall noise levels.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
In one aspect of the present invention, an apparatus is provided for converting a voltage signal into a current signal. A differential circuit is used to convert a voltage signal into a current signal. The differential circuit comprises at least one feedback amplifier to reduce non-linearity in the differential circuit.
In another aspect of the present invention, an apparatus is provided for converting a voltage signal into a current signal. The apparatus comprises: a first circuit capable of driving a signal onto a subscriber line; and a second circuit electrically coupled with the first circuit portion, wherein the second circuit comprising at least one feedback amplifier to reduce non-linearity when converting at least one voltage signal into a current signal.
In yet another aspect of the present invention, a method is provided for converting a voltage signal into a current signal. At least one voltage signal is converted into a current signal using a differential circuit comprising a plurality of transistors. A feedback is performed using a feedback amplifier to reduce non-linearity of the transistors during the conversion of the voltage signal.